How to find ridges in DEM image?

I have a DEM image, which shows low walls (like in old agriculture) and I try to extract those walls.

The problem is the ground in slope, so the left walls is in the same altitude as the right ground. It easy to see the ridges of the wall, but it hard gets them out.

Maybe someone has an idea?

Answer 1

[Todd Harris]

Image morphology holds some useful answers. I'm currently working on a project trying to detect mountain ridges and valleys and have a few half-baked methods ready to go that will produce several useful results. The one below follows the method described in an old paper (DOI: 10.1016/j.geoderma.2003.10.008 ). I highly suggest you check out the topotoolbox as well, it has a great function called excesstopography that essentially removes large scale gradients from a DEM, making it easier to find valleys and ridges that run up or down a larger hill/mountain.

function [SCHv,SCHr,SCHhs,SCHc] = Valley_SCH(DEM,rad,thr)
%VALLEY_SCH finds valleys and ridges from a DEM
%   Method follows that published by Schmidt and and Hewitt 2004 
%   ( DOI: 10.1016/j.geoderma.2003.10.008 )  
%
% ---INPUT---
%   DEM
% The digital elevation model
%   rad
% The radius of the structural element
%   thr
% The threshold for "noise removal"
%
% ---OUTPUT---
%   SCHv
% Valley map
%   SCHr
% Ridge map
%   SCHhs
% "Hillslopes" map (flat areas and smooth areas)
%   SCHc
% Combined map to show all four categories.

wb=waitbar(0,'Applying Schmidt method - Valleys...');
DEM(isnan(DEM))=0;
SE=strel('disk',rad,0);

VD=imbothat(DEM,SE);
VD=VD-thr;
VD(VD<0)=0;

waitbar(0.5,wb,'Applying Schmidt method - Ridges...')
HH=imtophat(DEM,SE);
HH=HH-thr;
HH(HH<0)=0;

waitbar(0.9,wb,'Applying Schmidt method - Finalising...')
%Valleys
SCHv=zeros(size(DEM));
SCHv(and(VD>0,HH==0))=1;
temp=VD./(HH+VD);
SCHv(and(VD>0,HH>0))=temp(and(VD>0,HH>0));

%Ridges
SCHr=zeros(size(DEM));
SCHr(and(HH>0,VD==0))=1;
temp=HH./(HH+VD);
SCHr(and(HH>0,VD>0))=temp(and(HH>0,VD>0));

%Hillslopes
SCHhs=zeros(size(DEM));
SCHhs(and(HH==0,VD==0))=1;
temp=2*min(HH,VD)./(HH+VD);
SCHhs(and(HH>0,VD>0))=temp(and(HH>0,VD>0));

%Combined
SCHc=int8(zeros(size(DEM)));
SCHc(and(VD>0,HH==0))=3;
SCHc(and(HH>0,VD>0))=2;
SCHc(and(HH==0,VD==0))=1;
SCHc(and(HH>0,VD==0))=0;
close(wb)
end